This invention relates in general to the field of electromagnetic signal circulators and in particular to active monolithic microwave integrated circuit (MMIC) circulators.
A circulator is a three-terminal device which passes signals input to one port to the next port in a rotational fashion (either clockwise or counterclockwise) without allowing signals to pass in the opposite rotation. Circulators are suitable for essentially any radio frequency (RF) application, including communications. Circulators are also useful as isolators, easily made by tying the third circulator port to ground through a resistor. Other applications involve radar (including phased array systems) and electronic counter measures (ECMs).
Microwave circulators can be accomplished passively or actively. Passive microwave circulation is accomplished using magnetics, or, at higher frequencies, waveguide magnetic structures. Both magnetic and waveguide magnetic techniques for microwave circulators suffer from relatively large physical size requirements. A passive magnetic circulator might comprise a volume five inches by five inches by two inches. Waveguides require even more space. Relatively high cost is associated with such relatively large physical size, and it has been especially uneconomical and impractical to use passive magnetic or waveguide microwave circulators in many applications.
Active microwave circulators can be realized in a relatively small physical space (e.g., approximately seventy mils square on a MMIC instead of the five inches by five inches by two inches for a comparable passive magnetic circulator). However, active circulators in general act as low pass devices, exhibit frequency limitations, and suffer from high insertion loss (e.g., insertion losses in the vicinity of 2.5 to 3.0 deciBels (dB) are typical). In addition, conventional active circulator topologies could benefit from enhanced noise performance.
The first techniques for active circulation demonstrated a bipolar design. While demonstrating multi-octave bandwidth capability, this approach has been restricted in operational frequency range and is limited to low pass transfer functions. Implementation of active circulators in a gallium arsenide (GaAs) monolithic configuration has also been accomplished. However, with high (e.g., greater than 6 dB) insertion loss and no feedback for stabilization, the gallium arsenide monolithic configuration is still limited in performance. Furthermore, neither technique allows for configuring the passband, frequency range, and operating bandwidth of the active circulators.
Thus, a practical, economical circulator which overcomes the size and cost constraints of passive magnetic or waveguide circulators and which also overcomes the low pass characteristics, high insertion losses, and frequency limitations of conventional active circulators is highly desirable. The circulator should employ a topology which allows for a configurable passband, frequency range, and operating bandwidth for increased versatility. The structure should be able to transcend the high noise figure exhibited by conventional active circulators for increased utility in a wide variety of applications.